errors.Is and errors.As
- Go course
- Beginner
- Lesson 85/90
- 4 min
Learn error wrapping and errors.Is, errors.As usage.
errors.Is and errors.As
Wrapped errors can't be compared with == because the outer error is a different value. To look inside the chain:
err := fmt.Errorf("profil: %w", ErrBulunamadi)
fmt.Println(err == ErrBulunamadi) // false
fmt.Println(errors.Is(err, ErrBulunamadi)) // true
var dh *DogrulamaHatasi
if errors.As(err, &dh) {
fmt.Println("alan:", dh.Alan)
}errors.Is searches for a specific error value, errors.As for a specific error type.
Comparing correctly
Rule: in error checks always use errors.Is instead of ==; then your code keeps working even if the error is wrapped later. The standard library does the same: errors.Is(err, fs.ErrNotExist), errors.Is(err, context.DeadlineExceeded), errors.Is(err, sql.ErrNoRows).
Practice Search the chain
Create ErrYok := errors.New("yok") and err := fmt.Errorf("katman 2: %w", fmt.Errorf("katman 1: %w", ErrYok)). Print err == ErrYok and errors.Is(err, ErrYok) on one line.
Starter code
package main
import (
"errors"
"fmt"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"errors"
"fmt"
)
func main() {
ErrYok := errors.New("yok")
err := fmt.Errorf("katman 2: %w", fmt.Errorf("katman 1: %w", ErrYok))
fmt.Println(err == ErrYok, errors.Is(err, ErrYok))
}Expected output
false true
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Why is a pointer (<code>&dh</code>) passed to <code>errors.As</code>?
- So it can write the found error into that variable Correct answer
- Because it's faster
- It isn't required
- For a nil check
Explanation: If As finds the requested type in the chain, it assigns the value to the target variable; that needs the target's address.
2 Search for the type
var dh *DogrulamaHatasi
if errors.____(err, &dh) {
fmt.Println(dh.Alan)
}var dh *DogrulamaHatasi
if errors.As(err, &dh) {
fmt.Println(dh.Alan)
}Explanation: errors.As finds the requested type in the chain and assigns it to the target.
3 Question and tool
| Term | Meaning |
|---|---|
Is this error ErrX? |
errors.Is |
Is this error of type *MyErr? |
errors.As |
Unwrap one level |
errors.Unwrap |
err == ErrX |
Breaks once wrapped |